Elfin by Quinn Loftis

Before tonight Cassie Tate’s biggest concerns were whether she could pass Algebra and how she was going to keep Elora, her best friend, from dressing her in a skimpy fairy costume for Halloween. Her feet were firmly planted in suburban reality and she had no reason to believe her life would be anything but that of a typical teenager. That is until tonight, when Cassie saw something that no human was ever supposed to see; in the blink of an eye she was thrown into the world of the Light and Dark Elves.
He comes from a realm where light and dark have fought for millennia. He is of a race known to humans only in myths and legends. The darkness that lives inside him is a part of both who and what he is and it makes him the most gifted spy and assassin in the history of his time. His life is not his own; he lives in the service of the Dark Elf King. He slays who he must, has mercy on no one, is relentless in his hunt, and never tires of seeing his prey fall. He is Triktapic, assassin, spy, most feared of the Dark Elves.
Now, in the midst of his King’s complicated plans to expand the Dark Elves’ holdings into the mortal realm, for the first time, Trik finds his loyalties divided. For no Elf, Dark or Light, can turn away from their Chosen.
Unbeknownst to the mortal realm, the battle between Light and Dark is being brought to their doorstep. The only one who can keep it at bay holds darkness in his heart like a lover, and the one who can sway that heart must decide if she can look beyond his black past, beyond his evil nature and see the man he is destined to be.
The questions must be asked, does love really cover a multitude of sins? Can true love actually conquer all or will his darkness consume those around him until all that stands is an assassin with the blood of the mortal realm on his hands?